1. Field of the Invention
The present invention relates to a lens driving module and, more particularly, to a piezoelectric driving module with a piezoelectric motor for driving a lens module, wherein the lens module, when so driven during a zooming or focusing process, is rubbed against a rubbing element so that a predetermined frictional force is generated therebetween to reduce a gravity-induced speed difference of the lens module.
2. Description of the Prior Art
Please refer to FIG. 1 for an exploded perspective view of a conventional focusing lens unit which includes a focusing mechanism 9 based on mechanical transmission. In order to drive the supporting seat 93 mounted with the lens set 92, the focusing mechanism 9 uses a costly precision driving element 91 (e.g., a step motor, a supersonic motor, etc.) and a large number of transmission elements. These driving and transmission elements not only increase the size, complexity, and cost of the entire mechanical structure, but also lead to a complicated assembly process, high power consumption, and consequently a high selling price.
With the continuous advancement of technology, the traditional photographic devices are being constantly improved in terms of image quality and compactness, so as to meet the diversified needs in the information age. For instance, the conventional step motors are being replaced by voice-coil motor (VCM)-based or piezoelectric motor-based drivers to further downsize the driving structure. In addition, there is a trend to integrate the functions of different products. The picture-taking function, for example, has been incorporated into mobiles phones, personal digital assistants (PDAs), and laptop computers to provide these products with more powerful visual image functionalities.
A piezoelectric motor is made of a piezoelectric material. When subjected to an applied voltage, a piezoelectric motor can generate an actuating force suitable for displacing a lens module to be focused or zoomed. More particularly, a piezoelectric motor produces a piezoelectric effect, which is a reversible process and can be divided into a “direct piezoelectric effect”, which refers to the generation of voltage by the piezoelectric motor due to a volume change of the motor material, and a “converse piezoelectric effect”, which refers to a volume change of the motor material triggered by an applied voltage. All materials exhibiting the aforesaid piezoelectric effects are called “piezoelectric materials”. In addition to quartz, tourmaline, Rochelle salt (potassium sodium tartrate), and like natural crystals, piezoelectric materials include those synthetically produced in the form of zinc oxide, polymers, ceramics, and composite materials. Piezoelectric ceramic is currently the mainstream of piezo members because of its small volume, fast response, small displacement, and low power consumption. More advantageously, piezoelectric ceramic is easy to manufacture, can be made into almost any shape, and shows a diversity of properties that vary with its composition.
As a piezoelectric motor twists in a wavy manner under an applied voltage and drives a lens module by friction, the actuating force (i.e., frictional force) between the piezoelectric motor and the lens module is not constant during the driving process but has a magnitude that fluctuates rapidly and repeatedly like a wave. Therefore, while the lens module is displaced vertically up and down, the force of gravity acting on the lens module plays a more prominent role in moments when the actuating force of the piezoelectric motor is relatively small. As a result, a speed difference occurs when the lens module is moved back and forth in a zooming or focusing process. More specifically, the piezoelectric motor tends to move slower when driving the lens module vertically upward than when driving the lens module vertically downward, for the piezoelectric motor has to overcome the force of gravity acting on the lens module. Experiment results show that, given the same applied voltage and other operating conditions, the ratio between the speed at which a piezoelectric motor drives a lens module upward (or forward when the lens module faces upward) and the speed at which the piezoelectric motor drives the lens module downward (or backward when the lens module faces upward) can be as high as 1:1.7. Such a large speed difference increases not only the difficulty in precisely controlling the position of the lens module, but also the circuit complexity and cost of the positioning module employed.